This invention relates to a valve assembly for controlling the supply of pressurized fluid to a fluid operated device, such as a fluid steering system from a main pump and a reserve pump.
Hydraulic steering systems are provided with a reserve pump in order to enhance operational safety. Automatic pump connecting valves are provided in such systems to assure the supply of fluid from the reserve pump in case the output of the main pump drops below a predetermined value. Such an automatic reserve pump connecting valve is disclosed, for example, in U.S. Pat. No. 3,882,678, wherein, the connection of the reserve pump to the fluid operated device is effected by producing a pressure differential between two pressure chambers of a valve actuator for the connecting valve.
In the foregoing type of pump changeover valve assembly, partial malfunction of the main pump results in slow displacement or "creep" of the valve toward its pump changing position. It is, therefore, an important object of the present invention to provide a valve assembly for two pumps alternately supplying fluid to a fluid operated device, wherein pump changeover occurs as a result of accelerated approach of, valve piston to its pump changeover position even against high opposing forces with minimal hysteresis and no momentary hangup.
A further object, in accordance with the foregoing object is to provide a valve assembly wherein operational overlap between the two pumps is prevented.
Yet another object of the present invention is to provide a valve assembly through which a pump changeover is effected without the functional disadvantages that would otherwise occur because of over-dimensioning of the valve passages in the valve assembly.